Transplant vasculopathy (TV) is a chronic vascular disease characterized by concentric intimal overexpansion composed of smooth muscle-like cells (SMLCs) and associated matrix. Unlike acute transplant rejection that can be effectively prevented and treated with the state-of-the-art immunosuppression regiments, at present there is no approved medical intervention to prevent or even delay the development of TV. Our preliminary data, for the first time, revealed that excessive proliferation/synthesis and impaired apoptosis of SMLCs is a cellular basis by which intimal overexpansion may develop. Remarkably, we observed that indoleamine-2,3-dioxygenase (IDO) derived from endothelial cells (ECs) has the potential to down-regulate SMLC proliferation/synthesis via upregulation of vSMC differentiation-specific gene transcription, and directly abolish the anti-apoptotic property of SMLCs. In complementary in vivo experiments, we found that graft endothelium is the primary site for endogenous IDO induction, and that donor organs (but not recipient animals) lacking this gene are more susceptible to TV lesions. In light of these novel data, we hypothesize that endothelium-derived IDO directs a reverse phenotypic switch of SMLC towards a more quiescent and differentiated state, and that specific overexpression of this enzyme in graft ECs blocks vascular remodeling in TV. To test this central hypothesis, we have developed a number of unique animal models and approaches, which include mouse models of heart or pulmonary TV and the Sleeping- Beauty (SB)-based nonviral gene integrating strategy capable of long-lasting human IDO (hIDO) transgene expression within graft endothelium. In Specific Aim 1, we will determine the effect of endothelial IDO on SMLC pro-proliferative/synthetic properties. In Specific Aim 2, we will determine the effect of endothelial IDO on SMLC anti-apoptotic properties. In these studies, the possible molecular mechanism(s) behind IDO-induced phenotypic switch of SMLCs will be explored. In Specific Aim 3, we will examine the role of enhanced endothelial IDO in the development and reversal of experimental TV. In this study, the possible beneficial effect of endothelial IDO on functional and structural vascular remodeling in graft will be assessed by echo and histological studies, respectively. Results generated from this project may open a new avenue for the prevention and treatment of this devastating disease, especially given that the SB-based nonviral approach is already advancing into clinical trials.